Objectives: The long-term virological efficacy of lopinavir/ritonavir-containing highly active antiretroviral therapy (HAART) in HIV-infected patients failing a first-line protease inhibitor (PI)-based regimen is still unclear. Methods: An observational study was carried out from December 2000-December 2002 on 111 consecutive patients starting lopinavir/ritonavir. The primary end-point was virological success (HIV RNA <50copies/mL in two consecutive determinations). CD4 outcome, lipid levels and adverse events were recorded. The Kaplan-Meier method and log-rank test were used to estimate the time-dependent probability of reaching the end-point using intention-to-treat and on-treatment approaches. Results: Ninety-six patients obtained virological success during follow-up; Kaplan-Meier analysis showed that the time-dependent probability of obtaining this end-point was 78.4% at month 12 and 85.8% at month 24. The median CD4+cell count increased by 118 cells/mm3 from baseline to month 12 and by 153 cells/mm3 to month 24. Thirty-one patients discontinued lopinavir/ritonavir: 16 because of drug-related toxicities, six for simplification, five because of virological failure, one patient was lost at follow-up and three died. An elevation in lipid parameters was observed, but only a minority of patients developed a grade 3 or higher hypertriglyceridaemia and/or hypercholesterolaemia. Among the 15 patients not reaching virological success, five had (less-than or equal to)5 mutations in the protease region known to reduce susceptibility to lopinavir /ritonavir (one discontinued lopinavir/ritonavir because of gastrointestinal intolerance), five had no mutations (two discontinued lopinavir/ritonavir because of gastrointestinal intolerance), and five showed (greater-than or equal to)6 mutations (all discontinued lopinavir/ritonavir); however, of the patients who discontinued lopinavir/ritonavir none achieved HIV RNA <50 copies/mL on subsequent regimens. Conclusions: Lopinavir/ritonavir was highly effective and well tolerated in HIV-infected patients failing a first-line PI-based HAART.